A projection-type color LCD device, though appending light sources to it is required since an LCD element does not emit light itself, has excellent characteristics such as having a wider color reproduction range, being compact and light, and not necessitating convergence adjustment, in comparison with projection-type cathode ray tube display devices. Therefore, further development of the projection-type color LCD devices is earnestly expected.
As projection-type color image display arrangements with the use of LCD elements, there are a triple-panel arrangement whereby three LCD elements are used so as to correspond to the three primary colors respectively, and a single-panel arrangement whereby a single LCD element is used. According to the former arrangement, an optical system for separating white light into color lights of the three primary colors, that is, red, green, and blue (hereinafter referred to as R, G, and B, respectively), and LCD elements for controlling the color lights so as to form images are independently provided. By optically overlapping the respective images of the colors, full color display is carried out. Since the light emitted from the white light source is efficiently utilized by this arrangement, a project-type color LCD device having high brightness can be realized. However, its optical system is complicated with a great number of parts, and hence it usually has a disadvantage in respect to cost and size in comparison with the single-panel arrangement, which is described below.
According to the single-panel arrangement, images of an LCD element having a three-primary-color filter having, for example, a mosaic pattern or a stripe pattern are projected by a projection optical system. An example of this arrangement is disclosed in the Japanese Publication for Laid-Open Patent Application No. 59-230383/1984 (Tokukaisho No. 59-230383). Since this arrangement has only one LCD element and an optical system simpler than that of the three-panel arrangement, it is suitable for a compact projection-type system which is produced at lower costs.
However, whereas the above-described arrangement has excellent features such as being produced at lower costs and being compact, it has a defect in that projected images tend to be dark, due to a decrease of utilized light which is caused by the color filter absorbing or reflecting the light. The utilized light is decreased in the single-panel arrangement to about one third of that of the three-panel arrangement using a light source having the same brightness.
To make the light source brighter can be thought of as an easy way to resolve the defect. However, there are still respective problems, in the case where the color filter is a light absorbing type and in the case where it is a light reflecting type composed of a dielectric mirror.
To be more specific, in the case where the color filter of the light absorbing type is used, light energy absorbed in the color filter turns into heat. Therefore, when the light source is made too bright, a temperature of the color filter rises, thereby not only causing a temperature rise of the LCD element but also causing discoloration of the color filter. Besides, how products of photochemical reaction of the color filter influence on liquid crystal is not completely elucidated, and it is anticipated that display defects due to the products may occur in the future, which leads to low reliability.
In the case where the latter light reflecting type, that is, the color filter composed of the dielectric mirror is used, the above problem of heat caused by making the light source brighter and the subsequent problem of discoloration are avoidable. However, since the dielectric mirror is patterned at a fine pitch, production costs remarkably rise. Thus, it is failed to achieve the advantage of the single-panel arrangement, that is, being produced at lower costs.
As a single-panel-type color LCD device wherein the brightness of projected images is improved, the Japanese Publication for Laid-Open Patent Application No. 4-60538/1992 (Tokukaihei No. 4-60538) discloses a device wherein dichroic mirrors 54R, 54G, and 54B are provided in a sector form, as illustrated in FIG. 12, so that white light from a white light source 51 is divided into respective light fluxes R, G, and B, for enhancing the luminous efficiency.
In this device, the light fluxes obtained by the dichroic mirrors 54R, 54G, and 54B respectively enter, at different angles, microlens array 55 provided on a light source side of an LCD element 57. The light fluxes having passed through the microlens array 55 are, in accordance with the respective different angles of incidence, respectively projected on sections of the LCD element 57. The sections of the LCD element 57 are respectively driven by signal electrodes to which corresponding color signals are independently applied. Since this device has neither the color filter of the light absorbing type nor the dielectric mirror, the luminous efficiency is improved, thereby making it possible to obtain bright images without sacrificing the advantage of the single-panel arrangement. Note that in FIG. 12, 59 represents a projection lens, and 60 represents a screen.
However, the device disclosed in the above-mentioned publication of Tokukaihei 4-60538 has a problem of lowering color purity. This problem is caused as follows: the light fluxes obtained by the separation by the dichroic mirrors are projected not on the corresponding pixels but on neighboring pixels, due to a low degree of parallelization of light in the case where a lighting means used therein has poor performance in respect to the degree of parallelization of light, aberration of the microlens, stray light caused by multiple reflection between the dichroic mirrors, or the like.
As an arrangement for restraining the lowering of the color purity, the Japanese Publication for Laid-Open Patent Application No. 7-181487/1995 (Tokukaihei 7-181487) discloses an arrangement wherein a color filter 62 as means for regulating wave length is provided at an entrance pupil position (pupil plane) of a projection lens 61, as shown in FIG. 13.
In a device of this arrangement, in the case where some light fluxes pass through pixels of wave length ranges which do not correspond to the wave lengths of the light fluxes, the light fluxes are blocked by the color filter 62 provided at the entrance pupil position of the projection lens 61. Therefore, even in the case where lighting means of poor performance in respect to a degree of parallelization of light is used, or even in the case aberration of the microlens or stray light due to multiple reflection between the dichroic mirrors occurs, undesired color mixture is prevented, while high-quality projected images having high color purity can be obtained.
In addition, usually the color reproduction range is determined depending on total characteristics of color separating means and color synthesizing means in the case of a device having the three-panel arrangement, whereas in the case of a device disclosed in the above publication Tokukaihei 4-60538, a color reproduction range of displayed images is virtually determined only by the dichroic mirrors as color separating means, which respectively correspond to the primary colors R, G, and B. Therefore, a satisfactory color reproduction range cannot be realized unless each of the dichroic mirrors has high wavelength selectivity, and hence the color separating means costs higher than that used in a usual projection-type color LCD device having the three-panel arrangement. But by thus providing the color filter 62 at the entrance pupil position of the projection lens 61, the problem of sensitive wavelength selectivity is solved.
Thus, in both the devices disclosed by the publications Tokukaihei No. 4-60538 and Tokukaihei No. 7-181487, brightness of projected images is improved in comparison with a device of the single-panel arrangement wherein an LCD element incorporating a color filter is used. However, both the devices have an arrangement wherein lights of the colors R, G, and B, projected on the LCD element at different angles, respectively, are converged by the microlens on respective corresponding apertures of pixels.
Therefore, the light fluxes having passed through the LCD element are diverged at angles each of which is a sum of each angle of incidence of the light fluxes on entering the LCD element 57 and a converging angle .theta.3 of a microlens 4a shown in FIG. 7. To catch the diverged light, a projection lens having a great aperture is necessitated. In other words, light entering the LCD element is converged at a converging angle .theta.3 so as to be converged at pixel apertures, while after passing through the pixel apertures, the light is diverged at a diverging angle .theta.3. Since such diverging light causes a large area to be irradiated, the projection lens has to have a great diameter so as to catch all the diverging light. Therefore, costs of the projection lens become higher than the costs in the case of the device of the usual single-panel arrangement. Thus, the lowering of the production costs cannot be desirably achieved.